Fix: avoid entries in Grid fields entries being shared between instances

[danielolsen]

Purpose

Fix a bug that was introduced in #121, where entries in the Grid.fields dict are shared between instances. Example:

>>> from powersimdata.input.grid import Grid
>>> western_grid = Grid(['Western'])
Reading bus.csv
Reading plant.csv
Reading gencost.csv
Reading branch.csv
Reading dcline.csv
Reading sub.csv
Reading bus2sub.csv
Reading zone.csv
>>> western_grid.plant.shape
(2527, 35)
>>> eastern_grid = Grid(['Eastern'])
Reading bus.csv
Reading plant.csv
Reading gencost.csv
Reading branch.csv
Reading dcline.csv
Reading sub.csv
Reading bus2sub.csv
Reading zone.csv
>>> western_grid.plant.shape
(10789, 35)
>>> eastern_grid.plant.shape
(10789, 35)
>>> (western_grid.plant == eastern_grid.plant).all().all()
True
>>> western_grid.plant is eastern_grid.plant
True

What is the code doing

In test_grid.py: adding a test that detects this behavior.

In grid.py: moving the definition of the fields and transform dicts into the __init__ so that they are no longer shared.

Time to review

Very quick. The test and fix are both very simple.

[rouille]

Wouah good catch.

[dmuldrew]

I see the issue...these fields were a properties of the class and it popped up when there was more than once instance of the grid. Sorry I've been translating my object design methodology from C# and I missed this Python subtlety...

C# => Python
class: static prop => prop
instance: prop => self.prop